The United States Department of Energy has a total of 1,936 radioactive cesium-137 (cesium) and strontium-90 (strontium) capsules, which are regarded as waste. The capsules are stainless steel containers collectively holding about 130 million curies of radioactive cesium and strontium. The cesium is in the form of cesium chloride and there are 1,335 of these capsules. The strontium is in the form of strontium fluoride and there are 601 of these capsules. The cesium and strontium are double encapsulated in two types of stainless steel tubes with welded end caps. For the cesium capsules, the inner capsule is 316L stainless and the outer capsule is 316L stainless. For the strontium capsules, the inner capsule is Hastalloy and the outer capsule is 316L stainless. 23 of the cesium capsules have an additional overpack. The outer dimensions of a cesium capsule is 6.67 centimeters (2.63 inches) in diameter and 51.05 centimeters (20.1 inches) in length and of a strontium capsule is 6.67 centimeters (2.63 inches) in diameter and 52.77 centimeters (20.78 inches) in length. For purposes of this disclosure, the waste capsules may be these exact dimensions or may be larger as a result of overpacking them in another container. Overpacking may be necessary because of any leakage or suspected leakage in a current capsule, or to increase confidence in environmental containment, or to enhance safety, or to simplify handling. Such overpacking may involve surrounding the capsule within bismuth or other metals within the overpack. Whether a cesium or strontium capsule exists as it is now packaged or it is overpacked with another capsule, the principle of the invention described herein is the same and the final capsule is referred to herein as a waste capsule, or simply a capsule.
There are two groups of capsules presently being stored. The first group of both cesium and strontium capsules was encapsulated before December 1983. The second group of strontium capsules alone was encapsulated after December 1983. The capsules have a high-thermal output and high-radiation dose rate and are stored in water-cooled pool cells at the Waste Encapsulation and Storage Facility at the Department of Energy's Hanford reservation in the State of Washington. Underwater storage removes heat and provides radiation shielding. The contents of the capsules are considered solid material.
The capsules have been identified as high-level mixed waste and disposal is subject to the Resource Conservation and Recovery Act regulations. The original planning assumption had been that the capsules would be transferred to a Waste Treatment Plant at the Hanford Site, mixed with high-level waste and then vitrified for subsequent disposal at the spent fuel and high level waste repository at Yucca Mountain, Nev. The Hanford Performance Management Plan Revision D, dated August 2002, calls for leveraging the existing safe configuration of the sealed cesium and strontium capsules to provide a permanent isolation pathway that does not require vitrification, thereby avoiding the risks associated with opening the capsules. Therefore, if a safe, simple and regulatory compliant means for disposition of the capsules could be implemented, it could have cost, safety and security benefits.
It is an object of this invention to use any standard Hanford vitrified high-level waste canister as the external container for packaging the capsules. This would facilitate disposal of the capsules at the repository. The standard Hanford vitrified high-level waste canister is described in the United States Department of Energy's Waste Acceptance Product Specifications, which are incorporated herein by reference. While the standard canister may change, the essence of the invention is to use whatever canister is the standard for vitrified high-level waste disposal. A basic principle of the invention is to provide a disposal package meeting the weight specification for high-level waste canisters. Since this regulatory weight limit is determined based upon the density of the vitrified waste within, using a disposal package applying the principles of the invention will create a disposal package meeting the regulatory weight limit. While there are other regulatory criteria to be met, the weight limit is a key critical concern when it is decided to use the same high level waste canister for a cesium or strontium capsule disposal package. So even if the size or dimensions of a standardized Hanford high-level waste canister are changed, the principles for making the cesium and strontium capsule disposal package remain the same.
Prior art describes an inner receptacle for holding waste within an outer receptacle. It teaches filling the space between the inner receptacle and outer receptacle with a mass of shielding material. If this design were used for a standard Hanford canister, it would cause the weight of the canister to exceed regulatory limits. It is an object of the present invention to meet the regulatory weight limit of 4,200 kilograms for the disposal package. Therefore, a significant improvement in existing technology is that the radiation shielding material does not fill an annular space between an inner receptacle and an outer container, but only a small hole bounded by the outer wall of the waste capsule and the inner wall of an inner container. Unlike all prior inventions, the walls of inner container fill most of the space within the outer container. Choosing an inner container lower than a specified density enables the disposal package to have a total weight less than the regulatory weight limit. In contrast to the instruction of the prior art, the inner container, that is the means for containing, is not chosen for its radiation shielding capability, but rather is chosen for its density, high melting temperature, and longevity of containment potential.
Prior art teaches the use of a sleeve within a radiation shielding material within an outer container. The sleeve surrounds, but does not encase, the waste assembly centering it and conducting heat in a desired path. As in the above example, radiation-shielding material occupies the space between the outer wall of the sleeve and the inner wall of the outer container, which would be unacceptable in terms of meeting the regulatory weight limit for the disposal package. This design improvement in the current invention reduces the volume of radiation absorbing substance to a minimum, that is, an amount required to fill a hole within a second container within the outer container. It, thus, significantly reduces the weight of a disposal package and enables the utilization of a standard Hanford vitrified high-level waste canister as the outer container in compliance with the regulatory weight limitation for repository disposal. Second, the partially encasing sleeve of the prior art is eliminated and instead the waste is encased in a thermally conducting material, which also serves to provide a necessary amount of radiation shielding to comply with repository disposal regulations. Encasement in thermally conducting material preserves the thermal conduction function and provides another high integrity container for the waste capsule.
Prior art teaches filling the outer container with a meltable heavy radiation shielding material, such as lead. The lead was typically placed between the outer container and the waste. At a density of 11.35 grams per cubic centimeter, filling with lead would result in a package too heavy for repository disposal. Lead also shrinks upon solidification and this potentially causes gaps between internal components within the disposal package. Gaps between internal components will interrupt heat transfer via thermal conductivity and could cause unacceptably high temperatures within the disposal package. The current invention is a substantial improvement to this type of prior art in several ways: Firstly, the disposal package weight is reduced to meet regulatory limits by mostly filling the outer container with an inner container made of relatively low-density material. Secondly, thermal conduction is maintained by using a shielding material that expands upon solidification and, thus, maintains physical contact between the waste capsule and the inner container. Thirdly, containment longevity is substantially improved by using a material for the inner container that has a lifetime measured in geologic time spans. And, fourthly, the prior art uses the molten lead as the encasing material between the outer container and the inner container. Should the outer container fail, the radiation shielding material, itself a hazardous substance would be exposed to the environment. The current invention seals the radiation shielding material, within the inner container and since the inner container is not made of a hazardous material, provides an added barrier to release of environmental contaminants. The present invention also uses materials for the shielding that are not listed hazardous. Thus, the filled disposal package is a waste capsule encased within a safe material, which is then sealed within a long-lived, low density, non-melting inner container, which is then sealed within the standard Hanford disposal canister. This combination of multiple high-integrity containments complies with the applicable disposal regulations.
It is an object of this invention to provide a safe, simple and regulatory compliant disposal package and method for making the package.